The "Big Three" Rounds are the Paddy Buckley Round (Wales), Bob Graham Round (Lake District), and Charlie Ramsay Round (Scotland). For a bit more background on the individual rounds see https://www.gofar.org.uk/uk-big-3-rounds. The route itself is not explicitly defined - the requirement is that all of the summits on each round are reached (113 total peaks). This requires about 190 miles of running with 84K feet of elevation gain. Note that unlike the individual rounds there are no restrictions on completion time for any of the rounds - it is all one effort and the total elapsed time (including transit between the Rounds) is what counts for the FKT.
John Kelly included cycling between the Rounds, adding about 400 miles of cycling, to create what he called "The Grand Round". Note that you do NOT have to close the loop on the bike! Kelly's first completion in 130h 43m consisted of ~18% of the total time spent cycling, ~62% running, and the remaining time spent on sleep, transitions between sections, and other stops.
Photos by https://www.thephbalance.co.uk/